Channel-type electron multiplier for use with display device

ABSTRACT

A television-type display which utilizes a special scanning mode in combination with a picture tube including a channel-type electron multiplier and a continuous primary electron source for all the holes therethrough. The electron multiplier has channels or holes across which two sets of insulated conductive strips extend. One set is perpendicular to the other. One strip of each pair is supplied with a voltage to allow only one hole at a time in the electron multiplier to emit electrons. Scan is thereby effected. Intensity may be controlled by applying a suitable voltage between perforate conductive layers bonded to opposite sides of the electron multiplier or the strips themselves. A semiconductive coating may be used on the internal surfaces of the holes of the electron multiplier to provide for large current pulses while maintaining a high gain. A unique double layer conductive electrode arrangement is provided at one end of the channels of said multiplier.

Unite States atent 1 1 3,634,712-

[72] Inventor Richard K. Orthuber 3,520,83] 7/1970 Trap 3 l3/l03 X Sepulveda, Calif. 3,387,137 6/1968 Adams 313/104 x [2]] Appl. No. 19,727 3,243 642 3/1966 Gebez 313/68 X Filed 1970 Primary Examiner Ronald L. Wibert f Assistant Examiner-R. .l. Webster Asslgnee lmematlfnal Telephme and Telegraph Att0rneys-C. Cornell Remsen, Jr, Walter J. Baum, Paul W.

Corporatmn Hemminger, Charles L. Johnson, Jr. and Thomas E. New York Kristofferson Original application Aug. 19, 1968, Ser.

No. 753,448. Divided and this application 1970, 1 ,727 ABSTRACT: A television-type display which utilizes a special scanning mode in combination with a picture tube including a channel-type electron multiplier and a continuous primary [54] CHANNEL'TYPE ELECTRON MULTIPUER FOR electron source for all the holes therethrough. The electron USE W DISPPAYPEVICE multiplier has channels or holes across which two sets of insu- 4 Claims 9 Drawmg Figs lated conductive strips extend One set is perpendicular to the 52 11.5. C1 313/103, othern strip f e h pair i u lied with a ltage to allow 313/104, 313/105, 315/10, 315/11, 315/12, only one hole at a time in the electron multiplier to emit elec- 313/ 7 trons. Sean is thereby effected. Intensity may be controlled by [51] Int. Cl. ..H0lj 43/00, applying a suitable voltage between perforate conductive H01] 31/26 layers bonded to opposite sides of the electron multiplier or [50] Field of Search 313/67, 68, the Strips themselves A semiconductive ing m y be d 103, 104, 105; 315/10, 1 l 12 on the internal surfaces of the holes of the electron multiplier to provide for large current pulses while maintaining a high [56] References Cited gain. A unique double layer conductive electrode arrange- UNlTED STATES PATENTS W ment is provided at one end ofthe channels ofsaid multiplier. 3,400,291 9/1968 Sheldon 313/105 X E l/ s 4'4 t PATENTED JAN] 1 1972 SHEET 2 UF 4 I no nlor e/c/y/wo e. 02/5/0552 CHANNEL-TYPE ELECTRON MULTIPLIER FOR USE WITH DISPLAY DEVICE This application is a division of copending application Ser. No. 753,448 filed Aug. 19, I968, and of the same title. The benefit of the filing date of said copending application is, therefore, hereby claimed for this application.

BACKGROUND OF THE INVENTION This invention relates to devices for displaying video signals, and more particularly to a novel photomultiplier configuration in the storage or picture tube of a television type display.

The invention will be found useful in many applications not disclosed herein. For example, the invention is not limited for use to a television picture tube, but may be employed with any other kind of suitable storage tube or device. Thus, the invention is not to be limited to any specific application disclosed herein. However, the invention will be found to possess considerable utility in a color television receiver.

In the past, kinescopes generally and especially picture tubes for color television receivers have had a multitude of complicated component parts which have been difiicult to adjust. Color TV picture tubes have also been relatively heavy and large in size. The shipping and storage space required for these tubes has thus also been large.

In a conventional color TV, the picture tube has three electron guns which selectively project three independent electron beams simultaneously through an aperture within a large set of such apertures contained in a so-called shadow mask. Electrons which pass through the mask illuminate a phosphor screen. The beams must necessarily be relatively long to cover the entire screen. However, the beams must all be focused and deflected. This situation is very sensitive and critical. Moreover, convergence problems are created which are difficult to solve, and stray magnetic fields of the earth can affect deflection and convergence. For example, a color TV receiver may be put out of alignment by moving it about in a room.

In accordance with the foregoing, conventional color TV does not give a highquality picture with stable operation unless the TV is made as a result of high-quality production. Further, a high-quality picture cannot be produced with any substantial degree of stability unless frequent and expensive servicing is provided.

The shadow mask keeps the electron beam illumination of the phosphor screen limited to mutually exclusive areas corresponding to the three beams. The shadow mask thus reduces the display brightness for a misalignment of any extent.

In a conventional color television display employing only one electron beam, devices to solve the focusing, deflection, convergence problems are even more complicated and expensive to manufacture and to maintain.

SUMMARY OF THE INVENTION In accordance with the device of the present invention, the above-described and other disadvantages of the prior art are overcome by providing a display for television receiver or the like for receiving a picture intensity control signal and a timing signal synchronous therewith from a transmitter. The receiver comprises a picture tube including a first device, for example, a phosphor screen and a source of primary electrons. The device of the present invention is especially characterized by a channel-type electron multiplier to receive primary electrons. The electron multiplier may be of the general type disclosed in U.S. Pat. No. 3,327,151. The electron multiplier has an output directed toward the first device. A first arrangement is then also provided which is responsive to the timing signal for producing an electron output from successive portions of the total area on the output side of the electron multiplier. A second arrangement is provided for controlling the intensity of the electron outputs from the said successive portions in synchronism with the operation of the first arrangement.

It is also an outstanding feature of the present invention that a special channel-type electron multiplier is provided to produce a large current with an accompanying high gain. This electron multiplier has a semiconductive layer capable of producing secondary emission. The layer is bonded to the internal surfaces of a plurality of holes in an insulator. The semiconductive layer has a conductivity intermediate that of the insulator and certain conductive layers which are employed to gate certain holes on and to control the intensity of the outputs of the electron multiplier.

In accordance with the foregoing, it will be appreciated that all the display tube components may be small and thin. A source of primary electrons may take several forms. A flat cold cathode may be employed, or a flat photocathode may be employed, if desired. Alternatively, the electron multiplier may be illuminated with the output of a flood electron gun. In any case, it will be appreciated that all the tube components including a source of primary electrons, the electron multiplier, and the phosphor screen may be very small and thin. Further, they may be located very close together. Substantially, no beam deflection or converging equipment is required. The device of the present invention may thus be made inexpensively of a few uncomplicated parts. By use of a close proximity focus between the electron multiplier and the phosphor screen, the picture tube contents of the present invention may be constructed in a manner to be housed in a very thin evacuated envelope.

The above-described and other advantages of the present invention will be better understood when considered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, which are to be regarded as merely illustrative:

FIG. 1 is a schematic diagram of apparatus employing one embodiment of the present invention;

FIG. 2 is a schematic diagram of apparatus employing a second embodiment of the present invention;

FIG. 3 is a schematic diagram of a third embodiment of apparatus employing the present invention;

FIG. 4 is a front elevational view of a portion of a channeltype electron multiplier according to the invention;

FIG. 5 is a front elevational view of the electron multiplier with certain conductive strips applied;

FIG. 6 is a front elevational view of the electron multiplier showing the relationship of two sets of conductive strips;

FIG. 7 is a sectional view of an electron multiplier constructed in accordance with a fourth embodiment of apparatus employing the present invention, taken on the line 77 shown in FIG. 5;

FIG. 8 is a sectional view of an electron multiplier constructed in accordance with a fifth embodiment of apparatus employing the invention; and

FIG. 9 is a sectional view'of an electron multiplier structure in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the drawings in FIG. 1, an evacuated envelope is indicated at 10 having a transparent window 11, a planar extended source of electrons 12, a channel-type electron multiplier 13 and a phosphor screen 14. Source 12, electron multiplier 13 and screen 14 are essentially identical in size with an area slightly larger than the desired display size. All three are also mounted in close proximity to each other. The space between source 12 and electron multiplier 13 is not critical and can be chosen from 10 to 1,000 mils. The spacing between electron multiplier 13 and screen 14 is of the order of several hundred mils.

Source 12 is preferably a thin, cold cathode such as is disclosed by A. Moschwitzer and S. Wagner in Phys. Status Solidi, Germany, Volume 4, N0. 2, pages 357-364 1964.

Intensity control and scan control 101 are shown in all FIGS. 1, 2, and 3.

In FIG. 2, an extended source of primary electrons is shown in an envelope 15 having transparent windows 16 and 17. A

thin film photocathode 18 is illuminated by a planar source of light 19. An electron multiplier 20 is provided identical to electron multiplier 13. A phosphor screen 21 is provided identical to phosphor screen 14.

Except for the phosphor screens and the electron multipliers, the component parts of the invention thus far described may be entirely conventional by themselves, although their arrangement is new. Further, light source 19 may be located inside envelope 15, if desired.

In FIG. 3, an evacuated envelope is indicated at 22 having a transparent window 23. Window 23 has a color TV phosphor screen 24 coated thereon. An electron multiplier 25 is located adjacent screen 24. Electron multiplier 25 may be identical to electron multipliers 20 and 13. A conventional flood electron gun 26 is provided to produce a beam 27 of Hood electrons to illuminate the input side of the electron multiplier. The channels are indicated at 28 in FIG. 4. The spacing of the channel axes corresponds to one-third the spacing of horizontal lines in the TV display.

For example, for a 525 TV line display and a 20 inch diagonal display size, the picture height is 12 inches. Twelve inches -30 centimeters and the line spacing is this 30 525 centimeters which is 0.057 centimeter or 22.8 mils. In this case the channel axes are spaced 22.83 mils which equals 7.6 mils. The channel dimensions are smaller, for example 4 to 5 mils. The electron multiplier slab or plate may be produced by conventional techniques such as the Fotoceram process developed by Corning Glass Works, Corning, New York. The F otoceram process has been applied to the production of shadow masks, and is therefore relatively inexpensive.

The electron multiplier plate is then provided with a series of coatings of both surfaces as indicated in FIGS. 5 and 7.

Electron multiplier 20 includes a dielectric or semiconductive slab. These channels are positioned in a square array as indicated in FIG. 4.

FIG. 7 shows a cut through the slab of FIG. 5. The cut is in a horizontal plane containing the axes of a row of channels. It shows two channels 29 separated by walls 30. The left hand surface is coated first with a contiguous metal film 31 deposited by evaporation of copper, chromium, nickel, or aluminum or other metals, so that the entrance or exit ends of the channels are not obstructed. The thickness of this metal film should be about 1/10 micron. On top of this film, a dielectric highly insulating spacer film 32 is deposited e.g., by evaporation of SiO or by surface anodizing of the metal electrode 31. Similarly, a set of metal strips 33 is evaporated on columns of channel apertures and one to two mils narrower than the channel spacings. The metal strips 33 are therefore insulated from each other as shown in FIG. 5.

A similar set of metal strips 34 is deposited on the opposite sides of the slab as shown in FIG. 7. The only difference between strips 33 and 34 are that the strips 34 are oriented in a perpendicular direction to strips 33 on the other side. Three channels are provided for the three primary colors used in color TV.

Phosphor screen 21 is spaced closely to channel plate 20 with a high-positive potential with respect to the facing surface of the channel plate applied, to permit proximity focusing of the channel plate output onto the phosphor screen. The embodiment shown in FIG. 2 is being described here in detail. However, it will be appreciated that the embodiment shown in FIGS. 1 and 3 may be constructed in an identical manner. The phosphor is applied in the form of parallel strips of width S, or very slightly less with alternating strips of blue, green and red phosphor similar to the phosphor forming trios of the shadow mask tube.

The phosphor screen is mounted with respect to the channel plate so that the finer grid of the top electrodes with strip width less than S is parallel and registered with the phosphor strips. The system of phosphor strips is aluminized in the conventional way.

The location of strips 33 and 34 is perhaps best illustrated in the elevational view of FIG. 6.

Returning once more to the electrode system applied to the channel plate, an alternate electron multiplier 35 is shown in FIG. 8. An additional view of this plate would be the same as shown in FIG. 6 with the vertical lines dotted and the horizontal lines solid.

FIG. 8 again shows two channels 36 with channel walls 37 and a contiguous metal electrode 38, with insulating layer 39 and an array of mutually insulated parallel vertical strip electrodes 40 on top. Another system of parallel strips 41 of about three times the width of the strips 40 is arranged on the same side of the channel plate, the two strip systems are mutually insulated by an insulating spacer layer 42 similar to layer 39. A conductive layer of electrode 43 is fixed relative to the output side ofelectron multiplier 35.

The gain ofa channel-type electron multiplier is determined by the difference in potential between electrodes on opposite sides of the electron multiplier. Hence, when strips are used on each side of the electron multiplier, no other electrode need be used. On the other hand, when two sets of strips are provided on one side of the electron multiplier, as shown in FIG. 8, the additional electrode 43 must be provided.

In the operation of the picture tubes of the present invention intensity control may be accomplished in a great many ways. This control may be applied directly to the same strips which gate each channel hole individually on and off. The intensity control may be applied to electrode 43 or any other similar electrode. The intensity control may be applied to source 12, photocathode 18, light source 19, beam current control electrode, not shown, of flood gun 26.

Although it is possible to control the intensity of each individual hole by making the number of rows of strips equal to the number of columns thereof, for black and white television, it is not necessary to triple the switching pulse repetition frequency for the exemplary three colors of color television. Note will be taken that switching may take place as indicated in ELFA New Electroluminescent Display," Proc. IRE, Oct. 1958, pp. 1694 to 1699. See FIG. 12. The aplitudes of the X-coordinate pulses are determined by a video intensity control signal. The "pulse actuated switches and intermediate storage cells" (PAM) are simply devices to amplitude modulate the X-coordinate pulses. To convert to color, simply use the two or three conventional color intensity control signals and connect each, in turn, to one to every third PAM.

Scanning is accomplished by gating off all of the channels except three channels at a time. Gating is accomplished simply by the use of a clock pulse generator, not shown, operating two counter registers having binary bits connected to corresponding strips. Such scanning is entirely conventional and will not be described. Such a scanning system may be of the type used in connection with solid state displays.

In the operation of the device of the present invention, application of a potential difference of approximately 1,000 volts to electrodes 38 and 43 creates an electric field within electron multiplier 35 such that electrons entering the channels from the input side are accelerated toward the phosphor screen. Repeated impacts of these primary electrons on the channel walls cause successive multiplication of the electrons within the channel walls as variously described in the literature, e.g., J. Adams and B. W. Manley, Electronic Engineering, March, 1955 pg. lO8M18l and G. W. Goodrich and W. C. Wiley, Review of Scientific Instruments, 33 page 761,

In this way, an electron current substantially above the emission density of the source of primary electrons is available to excite the phosphor screen. Due to the high field generated by the electron multiplier and screen by application of a potential of 20 to 25 kv to the phosphor screen, the individual channel outputs are proximity focused on the screen to establish a display of substantially the same resolution as that given by the reciprocal of the spacing of the channels in the electron multiplier.

An alternative electron multiplier 44 is shown in FIG. 9. Electron multiplier 44 is suitable for an emission of highcharge pulses and, at the same time, it is possible to accomplish highsgain multiplication. FIG. 9 shows a section of a channel plate with an insulating base 45, e.g., a plate formed by the well-known Fotoceramic process with etched channels.

On the input side a metal accelerating electrode 46 is deposited, e.g., by evaporation. Electrode 46 corresponds to electrode 31 shown in FIG. 7. Another metal electrode 47 is deposited on the output side. In contrast to the corresponding electrode 43 shown in FIG. 8, the electrode 47 is deposited to a considerable depth of the channel, e.g., one-fourth to onehalf the channel length. Suitable processes to accomplish this are condensation of highly colluminated vapor beams aligned with the channel axis. Preferably, the metal chosen for electrode 47 is readily surface oxidized by anodization or baking in an oxidizing atmosphere. Aluminum or nickel are suitable the latter being applicable by known treatments in an atmosphere of nickel carbonyl.

A highly insulating layer of metal oxide 48 is then formed electrically or by baking an oxidized atmosphere on top of electrode 47. A metallic contact electrode 49 is then deposited by evaporation under a grazing angle so that is has only very shallow penetration in the channel, i.e., about one channel diameter deep. A semiconductive secondary emissive sleeve 50 is deposited inside the channel which bridges electrodes 46 and 49 on both channel ends but is insulated by layer 48 from the part of electrode 47 which penetrates deeply into the channels.

One'way of depositing sleeve 50 is to introduce a suspension of suitable frit glass into the perforations of the channel plate, to spin out the excess of the suspension then to bake the plate above the melting point of the frit, which in flowing will form a smooth continuous coating on the channel walls. Glass of suitable composition is then, by hydrogen firing, adjusted to the desired conductivity.

ln operating the electron multiplier 44, a high-accelerating potential is applied to electrodes 46, 47 and 49. The electrode on the output side will then be positive. With no electron current flowing through the channel or with an electron input small enough to prevent wall saturation anywhere in the channel, the potential distribution will be uniform as set up by the resulting uniform bleeder current along the coating 50 just as in any nonsaturating conventional channel amplifier. However, in the section of the channel into which electrode 47 penetrates, the wall will now be formed by the inner electrode of a cylindrical capacitor, the oxide layer 48 as dielectric. The capacity density of this electrode may be far higher than that of a similar wall section against the electrodes on the surface ofa conventional channel plate, Consequently, a pulse with an intensity and duration which in a conventional channel multiplier would lead to an intolerable gain-limiting distortion of the potential in this section, will now have negligible effect on the wall potentials and thus the internal field in the channels. In this way, electron pulses of high charge may be generated with a gain approximately that in an unsaturated conventional channel multiplier without electrode 47, but otherwise having similar properties.

The depth to which electrode 47 has to penetrate into the channel is given by the distance from the channel plate output surface of that channel section at which saturation effects become first noticeable and is, therefore, a function of input pulse amplitude, pulse duration, gain per unit channel length and bleeder current.

Some remarks concerning the thickness and structure of the insulating layer 48 seem in place here. For the case of deep penetration of electrode 47, e.g., one-half of the channel length, the potential difference between 50 and 47 near the inner termination of 47 would assume the value V,,/2 in practice about 500 volts. In this case, the dielectric 48 has to have a considerable thickness of about 40 microns to avoid dielectric breakdown, a thickness which would not be readily produced by the above-recommended oxidation procedure.

This process will, therefore, be more suitable for electrodes 47 with relatively shallow penetration. For deep penetration,

it is preferable that the insulating layer be formed by slurry coating of the channel wall with a frit of a glass of high-dielectric strength and a composition which is not subjected to change in the subsequent H firing process applied to the coating 50. This modified channel structure is then formed in the following steps:

1. Deposition of the penetrating" electrode 47 by condensation of metal vapor in high vacuum of metal organic reaction within the perforations of the insulating (e.g. Fotoceramic) base plate.

2. Application of insulating coat 48 in this case along the entire channel length from a slurry of high-dielectric strength glass frit.

3. Evaporative application of end electrodes 46 and 49.

4. Application of inner coating 50 from a slurry of reducible glass frit.

5. Hydrogen firing to establish proper conductivity in the coating 50.

The channel structure described above, combining the capability of emitting electron pulses of high-charge content and at the same timedue to its high S.E. gain-being able to operate with very low-current density inputs, meets the requirements for kinescopes of the present invention.

Although the foregoing TV picture tube and electron multiplier have been described in connection with color television, it is to be understood that the invention is by no means limited thereto, the invention being equally applicable to black and white television.

As stated previously, intensity control voltages may be applied to the parallel conductive strips, to other electron multiplier electrodes or conductive coatings, to light source 19, to the cathode, not shown, of flood electron gun 26, or to any other electron source.

The picture tube and electron multiplier of the present invention is, further, not limited to the use of conductive strips to gate the outputs of three or one channel or hole of the electron multiplier on and off. Any other means may be employed to do so. Furthermore, such means may be or may not be conductive strips or the like bonded to or not bonded to the electron multiplier dielectric.

NOte will be taken that the channel-type electron multipliers shown herein have circular holes therethrough. Although the circular cross section of a hole is common and preferred, this construction is immaterial to the invention and any other conventional hole cross section may be employed without departing from the invention. Further, such holes are normally perpendicular to the input and output sides of the electron multiplier, but such need not be adhered to stringently in accordance with the present invention.

It is to be noted that the television receiver and electron multiplier of the present invention may be used in television receivers, but are not limited thereto. The invention may thus be applied to storage tubes or any other apparatus.

Although electrode 43 is required in the embodiment of FIG. 8, such an electrode is not required at 31 in FIG. 7 and may be omitted, if desired.

In accordance with the foregoing, it will be appreciated that all tube components may be made extremely small and very thin. The source of primary electrons may take the form of a thin, cold cathode as indicated in FIG. 1. Alternatively, photocathode 18 may be employed with light source 19 as shown in FIG. 2, Further, flood electron gun 26 shown in FIG. 3 may be employed. In any case, it will be appreciated that all the tube components including the said sources of primary electrons, the electron multiplier, and the phosphor screen may be made very small and very thin. Further, they may be located very close together. No beam deflection or converging apparatus is then required. The device of the present invention may thus be made inexpensively of a few uncomplicated component parts. By use of the proximity focus between the electron multiplier and phosphor screen, the picture tube components of the present invention may be constructed in a manner to be used in a very thin evacuated envelope.

Although only a few specific einbodiments of the invention I have been illustrated and described, the invention is by no 'means limited to thoseembodiments selected for this disclosure. The invention should therefore, not be limited to such embodiments, the true scope of the invention being defined only in the; appended claims,

I claim: 1. A channel-type electron multiplier comprising: an insulator having .two opposite surfaces; a main conductive layer bonded to each of said surfaces, said insulator havinga plurali- I .ty of holes therethrough, both of saidlayers having a plurality l of holes .in alignment with each other and in alignment with a plurality of said insulator holes; a semiconductive layer capable of producing secondary emission at a ratio to primary emission greaterthan unity, said semiconductive layer being bonded to the internal surfaces of a plurality of said aligned insulator holes; said semiconductor having a conductivity inter mediate that of said conductive layers andthat of said insulator; and means comprising a first auxiliary conductive layer on said insulator underlaying said semiconductor layer, said first auxiliary conductive layer extending from one of said opposite surfaces and within said holes therein. an insulating layer over said first auxiliary layer, and a second auxiliary conductive layer extending over a portion of said insulating layer at said one of said opposite surfaces extending a short distance into said holes.

2. Apparatus as defined in claim 1 in which said semiconductivc layeris defined as substantially channels insaid insulator. i

3. Apparatus according to claim 2 in which said semiconductive layer is electrically joined to said main conductive continuous within the layer adjacent at least a portion of said holes along one of said opposite surfaces and said second auxiliary conductive layeris electrically joined tosaid semicolnductive layer adjacent to at least a portion of said holes alongthe other of said opposite faces.

4. Apparatus according to claim 3 in which said first auxiliary conductive layer extends a greater distance into each corresponding hole than ,does'said second auxiliary conductive layer, and said insulating layer extends a sufficient distance .into each of said holes to electrically insulate said ssemiconductive layer from said first auxiliary conductive layer. 

1. A channel-type electron multiplier comprising: an insulator having two opposite surfaces; a main conductive layer bonded to each of said surfaces, said insulator having a plurality of holes therethrough, both of said layers having a plurality of holes in alignment with each other and in alignment with a plurality of said insulator holes; a semiconductive layer capable of producing secondary emission at a ratio to primary emission greater than unity, said semiconductive layer being bonded to the internal surfaces of a plurality of said aligned insulator holes; said semiconductor having a conductivity intermediate that of said conductive layers and that of said insulator; and means comprising a first auxiliary conductive layer on said insulator underlaYing said semiconductor layer, said first auxiliary conductive layer extending from one of said opposite surfaces and within said holes therein, an insulating layer over said first auxiliary layer, and a second auxiliary conductive layer extending over a portion of said insulating layer at said one of said opposite surfaces extending a short distance into said holes.
 2. Apparatus as defined in claim 1 in which said semiconductive layer is defined as substantially continuous within the channels in said insulator.
 3. Apparatus according to claim 2 in which said semiconductive layer is electrically joined to said main conductive layer adjacent at least a portion of said holes along one of said opposite surfaces and said second auxiliary conductive layer is electrically joined to said semiconductive layer adjacent to at least a portion of said holes along the other of said opposite faces.
 4. Apparatus according to claim 3 in which said first auxiliary conductive layer extends a greater distance into each corresponding hole than does said second auxiliary conductive layer, and said insulating layer extends a sufficient distance into each of said holes to electrically insulate said semiconductive layer from said first auxiliary conductive layer. 